Aging resistant cold rolled sheet products

ABSTRACT

A STEEL COMPOSITION CAPABLE OF BEING CONTINUOUSLY CAST AND FURTHER PROCESSED TO PRODUCE DRAWING-QUALITY SHEET STEEL HAVING SUBSTANTIAL RESISTANCE TO AGING COMPRISING: 0.01-0.08% CARBON, 0.20-0.60% MANGANESE, 0.03-0.08% SILICON, UP TO 0.015% ALUMINUM, LESS THAN 0.01% BORON AND OTHER USUAL STEELMAKING IMPURITIES SUCH AS NITROGEN AND OXYGEN. IT IS ESSENTIAL THAT SUFFICIENT BORON BE INCLUDED TO PROVIDE A BORON TO NITROGEN RATIO OF 1.4 TO 2.5 WHEN THE OXYGEN CONTENT EXCEEDS 150 P.P.M., OR A RATIO OF 1.0 TO 1.4 WHEN THE OXYGEN CONTENT IS LESS THAN 150 P.P.M. IN FURTHER PROCESSING THE STEEL TO SHEET PRODUCTS, IT IS ESSENTIAL THAT THE HOT ROLLED PRODUCT BE COILED AT A TEMPERATURE ABOVE 1100*F.

United States Patent O 3,725,143 AGING RESISTANT COLD ROLLED SHEET PRODUCTS Harry M. Alworth, Monroeville Borough, and Sumner H. Kalin, Wilkinsburg Borough, Pa., assignors to United States Steel Corporation No Drawing. Filed Feb. 3, 1971, Ser. No. 112,444

Int. Cl. C22c 39/00 US. Cl. 148-36 7 Claims ABSTRACT OF THE DISCLOSURE A steel composition capable of being continuously cast and further processed to produce drawing-quality sheet steel having Substantial resistance to aging comprising: 0.01-0.08% carbon, 0.20-0.60% manganese, 0.03-0.08% silicon, up to 0.015% aluminum, less than 0.01% boron and other usual steelmaking impurities such as nitrogen and oxygen. It is essential that sutficient boron be included to provide a boron to nitrogen ratio of 1.4 to 2.5 when the oxygen content exceeds 150 p.p.m., or a ratio of 1.0 to 1.4 when the oxygen content is less than 150 ppm. In further processing the steel to sheet products, it is essential that the hot rolled product be coiled at a temperature above 1100" F.

BACKGROUND OF THE INVENTION For most applications, flat rolled steel products such as sheet, strip and tinplate are usually produced from ingot cast rimmed steels. Rimmed steels are preferred because of their superior surface quality and high degree of ductility. It is well known in the art that rimmed steel ingots are produced by casting a low-carbon, non-deoxidized steel into an ingot mold, where the decreasing temperature, and resulting decrease in oxygen solubility, causes the released oxygen to react with dissolved carbon and violently evolve carbon monoxide gas. This violent gas evolution called rimming action causes the ingot to solidify with a high purity, dense surface or rim and exceptional cleanliness and ductility throughout.

Rimmed steels do, however, have one serious disadvantage for some applications in that the steel products rolled therefrom are subject to a high degree of strain aging. That is to say, the temper-rolled steel undergoes a spontaneous increase in hardness and decrease in ductility upon prolonged storage even at room temperatures, which is primarily due to segregation of nitrogen. atoms to dislocations producing pinning of the dislocations by solute atoms. For those applications where prolonged storage or severe cold forming are expected, non-aging steels can be produced by casting the ingot with a steel that has been deoxidized with aluminum and/or titanium.

The steel compositions customarily used for the production of rimmed steels and aging-resistant or non-aging steels for sheet applications do not lend themselves to continuous casting processes. The rimming action which is essential to a quality ingot cast rimmed steel is severely detrimental in a continuous cast strand. In a continuous casting process, any gas evolution of more than minute quantities in the mold causes cavities and blow-holes within the casting because the gas does not have an opportunity to escape from the continuously formed strand as it has in the case of a conventional ingot. On the other hand, the continuous casting of any steel which has been deoxidized with aluminum causes the formation of an aluminum oxide scum on the surface of the molten metal within the mold. Particles of this scum not only become entrained within the casting to yield a dirtier product, but also tend to be concentrated on the casting surface which is highly detrimental to the good surface quality essential to most flat rolled steel products. It is commercially essential, H

3,725,143 Patented Apr. 3, 1973 therefore, in the continuous casting of slabs to be further rolled to thin products that steel compositions be used which will cause little or no gas evolution in the mold and, at the same time, will not provide an oxide scum which will contaminate the casting, so that a clean and ductile casting will be produced with good surface qualities.

U.S. Pat. No. 3,412,781 issued Nov. 26, 1968 to]. 'H. Richards discloses a novel process for high tonnage rate continuous casting of high quality slabs for further processing to sheet products. The crux of that invention resides in the fact that continuous cast slabs, having the qualities of a rimmed steel, can be produced by utilizing a steel having the composition: 0.01 to 0.08% carbon, 0.20 to 0.60% manganese, 0.02 to 0.08% silicon, and up to 0.015% aluminum with the balance of iron and incidental impurities. When continuously cast, the resulting slab has all the characteristics of a high quality rimmed steel. Although the above discussed invention has solved the problems associated with the continuous casting of slabs to replace rimmed steel ingots, the resulting product, like conventional ingot cast rimmed steels, is highly susceptible to aging. Although many attempts have been made to develop compositions or processes which will permit the continuous casting of a substitute for non-aging ingot cast steels, none of the results have been entirely successful, except when using specialized nozzles and fluxes. Utilization of the known deoxidizers such as aluminum, titanium, zirconium, and others in sufficient quantities to yield a non-aging product, have always created scum problems which result in a dirtier product and poor surface quality. Hence, continuous cast non-aging steels of suitable quality for thin flat rolled products such as sheet have not heretofore been readily available.

SUMMARY OF THE INVENTION This invention concerns our discovery of a unique steel composition which can be continuously cast to produce a non-aging or aging-resistant steel having exceptionally good quality for further processing to thin flat rolled steel products. The inventive concepts not only involve the critical control of steels composition, particularly a very critical balance between the boron, nitrogen and oxygen contents, but also the critical control of the hot-rolling finishing temperature, coiling temperature, and annealing temperature when further processing the steel to thin flat rolled products.

An object of this invention is to provide a unique steel composition which can be continuously cast and further processed to produce non-aging or aging-resistant highquality thin fiat rolled steel products with surface quality equal to that of conventionally produced rimmed steel.

Another object of this invention. is to provide a unique steel composition which can be continuously cast and further processed to produce high-quality thin flat rolled steel products having improved aging-resistance.

A further object of this invention is to provide a process for producing a non-aging drawing-quality sheet steel from a continuous cast steel slab.

Still another object of this inventon is to provide a process for producing a drawing-quality sheet steel having enhanced resistance to aging.

These and other objects and advantages are fulfilled by this invention as will become apparent from the following detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to one embodiment of this invention, the composition of a molten steel is determined and adjusted to provide a composition substantially as described in US. "Pat. No. 3,412,781, supra, namely, 0.01 to 0.08% carbon, 0.20 to 0.60% manganese, 0.03 to 0.08% silicon andup to-0.0l aluminum. As disclosed in the aforementioned patent, this composition will provide a steel melt which can be continuously cast to provide a slab having good mechanical properties and especially suited for fiat-rolling to sheet products, the sheet products being comparable-to those previously rolled from rimmed steel ingots.'As previously noted, however, this steel'is subject to aging, so that sheet products rolled therefrom are not suitedto prolonged storage. The present invention overcomes the. aging problem inthe steel by further adjusting the melt composition to contain a carefully controlled amount of boron not exceeding about 0.01%.

. to sheet, will cause the steel sheet to be too hard for many Specifically, the boron content must be 1.4 to 2.5 times the :nitrogen content, in weight relationship, in a conventional non-degassed steel; and on the other hand, must be 1.0 to about 1.4 times the nitrogen content, in weight relationship, in a steel which has been degassed to contain less than about 150 p.p.m. of oxygen.

In addition to the above critical composition controls, production of an optimum non-aging or aging-resistant sheet steel product is further dependent upon the careful critical control of the hot rolling procedures to which the continuous cast slab is subjected. Specifically, the continuous cast slabs, having the above composition, must be heated to a temperature within the range 2100 to 2300 F., for hot rolling, as is the customary prior art procedure. In order to effect suitable non-aging or agingresistant characteristics, it is essential that the hot rolling finishing temperature be above 1500 F. and preferably within the range 1550 to 1650 F., and that the hot rolled steel be coiled at a temperature above 1100 F. and preferably 1150 to 1250 F. Thereafter, the steel may be pickled, and cold rolled in accordance with conventional mill practices.

Considering the composition limits in more detail, the carbon content of the melt should not be less than 0.01%, and preferably not less than 0.03%, because otherwise the oxygen content of the steel would be too high for continuous casting. In addition, the lining life in the steelmaking furnace would be significantly shortened if the carbon content of steels therein are below 0.01%. At the other end of the range, the carbon content should not exceed 0.08% to assure sufficient ductility in the final sheet product.

The manganese and silicon ranges for the molten steel as noted above are preferred because of the synergistic effect of these amounts in preventing pinhole porosity of a steel whose carbon content is 0.01 to 0.08%. In addition, the oxygen content of the steel can be more easily estimated and controlled when manganese exceeds 0.20%.

The amount of acid soluble aluminum in the steel is preferably not greater than 0.015% because larger amounts tend to cause the formation of excessive quanticold forming operations such as press forming or deep drawing. To avoid excessive hardness, therefore, the boron content of our steel must not exceed about 0.01%. The mere adding of .a prescribed, amount of boron in the range of 0.001 to 0.010%. is not enough, however, to assure that aging properties will even be affected, since the steels boron content must further be very carefully controlled with respect to thesteels nitrogen content, andto some extent with respect to the oxygen content as well. In conventional non-degassed steels as described above, wherein oxygen contents would exceed 150 ppm, the boron content must be from -1.4 to 2.5 times the nitrogen content. Boron additions insufiicient to meet the minimum requirement of 1.4 times the boron content, even ratios of 1.3, will provide very insignificant, if any, improvement in the steels aging resistance, while ratios of 1.4 or greater will have a very substantial effect in promoting aging resistance. Hence the minimum ratio of 1.4 is indeed critical. At the other end of the range, ratios above the maximum limit of 2.5 do not, however, adversely affect the steels aging-resistance, and therefore,

. comparable aging properties can be achieved even with ties of non-metallic alumina inclusions. Furthermore, the

presence of alumina in large amounts is particularly undesirable because the metal oxide inclusions containing large amounts of alumina tend to form massive agglomerates rather than glassy films along the side walls of the mold as the casting descends. These massive agglomerates are very difficult to remove by the action of the cooling water spray below the mold, and mar an excessively large portion of the casting surface area, so that excessive slab conditioning is required. They can also cause uneven,

cooling in the mold and, therefore, increase the possibility of a break-out during casting.

boron to nitrogen ratios above 2.5. Nevertheless, an upper limit 2.5 on the boron to nitrogen ratio was arbitrarily chosen because excessive amounts of boron, i.e., more than about 2.5 times nitrogen when the nitrogen content is within the normal range of 0.003 to 0.005, will cause the resulting sheet steel to be quite hard, having a substantially adverse effect on the formability of the sheet. On the other hand, if the boron to nitrogen ratio is maintained within the more preferred range of 1.4 to 2.0, the resulting steel will have a ductility equal to or greater than comparable prior art steels.

When the steel has been degassed to oxygen levels of less than about ppm, we have learned that the minimum boron to nitrogen ratio of 1.4 is no longer applicable, and in fact, ratios below 1.4, i.e., about 1.0 to 1.4 are equally effective at improving aging resistance. Moreover, in such degassed steels, boron to nitrogen ratios above about 1.4 are actually detrimental, causing proportionally greater hardness at ratios thereabove. For degassed steels, therefore, or any steel having an oxygen content of less than about 150 p.p.m., the boron to nitrogen weight ratio should be from about 1.0 to 1.4. Less boron than about 1.0 times nitrogen content will become exceedingly less effective in promoting aging resistance, while boron in amounts exceeding about 1.4 times nitrogen will proportionally decrease product ductility.

C percent .03.06 Si do w .02 Mn do 01-025 0 p.p.m 600-900 S ..perc ent. .02 P ....percent 0.0l5

The standard basic oxygen furnace practice for making low carbon steel may be used without modification. However, it is frequently advantageous to modify the cus .tomary .BOP furnace practice by charging enough manganese to the furnace to obtain a residual manganese content of at least 0.1% in the furnace melt. It is essential that the residual manganese content in the furnace melt beat least 0.10% when the sulfur content of the iron supplied to the furnace is in a normal range of from about 0.025% to 0.050%, in order to keep the sulfur content in the furnace melt down to an acceptable amount not greater than 0.02%. Residual manganese contents of over 0.1% are obtained by the addition of a manganese ore to the furnace charge, or by the addition of hot metal (iron from the blast furnace) containing enough manganese to give the residual manganese content of at least 0.10%. The use of manganese ore is preferred, since high manganese hot metal usually contains so much phosphorus as to raise the quantity of phosphorus in the steel casting above acceptable limits. The use of manganese ore makes it possible to obtain the desired residual manganese content in the furnace melt without also obtaining an excessively high phosphorus content. Either a high grade or low grade manganese ore may be used. The amount of ore added is at least about 0.1% by weight of Mn, based on the total weight of the furnace charge. Generally larger quantities are required because a large part of the manganese is lost to the furnace slag.

The temperature in the furnace is customarily held within the range of 2850 to 3000" Temperatures above 3000 F. are to be avoided because these high temperatures cause rapid deterioration of the furnace lining, resulting in the presence of excessive quantities of refractory oxide slag in the furnace melt.

' It is impossible to produce a furnace melt having the desired composition for introduction into a continuous casting mold according to the conventional process. The equilibrium relationships existing between carbon and silicon at the usual furnace temperatures require that either the carbon content be above the acceptable maximum of 0.08% or that the silicon content be below the minimum of 0.03%. It is necessary to form a furnace melt having the desired carbon content (which cannot be satisfactorily reduced in the molten steel after it has been tapped from the furnace without vacuum degassing) and to add silicon as required to bring up the silicon content to the desired level. It is also necessary to add manganese to bring up the content to the level desired for the purposes of this invention. A large portion of the manganese content of the molten steel introduced into the mold is added after tapping of the furnace melt, because it is impractical to charge enough manganese to a basic oxygen process furnace to furnish the desired manganese content in view of the excessive losses of manganese to furnace slag.

Manganese may be added in the ladle in the form of silicomanganese, high or medium carbon ferromanganese, or electrolytic 1 manganese. The addition of silicomanganese also supplies the entire quantity of silicon which must be added in order to bring the molten steel composition up to the desired silicon level of 0.03-0.08%. Customarily about 6 to 10 lbs. per ton of silicomanganese and about 2 to 4 lbs. per ton of medium carbon ferromanganese are added in order to supply the necessary manganese and silicon to the molten steel. Instead of adding medium carbon manganese, either high carbon ferromanganese or electrolytic manganese may be added. Frequently the amounts of high carbon ferromanganese required are somewhat less than the amounts of medium carbon ferromanganese normally required, being only about 1 to 2 lbs. per ton in most instances.

The silicomanganese and the ferromanganese are most conveniently added to the molten steel during the filling of the tapping ladle with the furnace melt obtained in the steelmaking furnace. Best results are obtained when the silicomanganese and ferromanganese are added durin the filling of the middle third of the ladle.

In addition to manganese and silicon, it is frequently preferably, following the aluminum addition after the aluminum has gone into solution, but early enough to prevent the boron from rising in the steel and contacting the slag and thus becoming oxidized and losing its effectiveness. Any practical form of boron may be used, such as ferroboron, calcium boride, silicomanganese boron, and so on. If the steel is not to be degassed or vacuum treated so that normal oxygen contents exceeding 150 p.p.m. will be present, then as explained above, the boron addition should be sufiicient to provide a boron to nitrogen weight ratio of 1.4 to 2.5 and preferably 1.4 to 2.0. On the other hand, if the steel is to be degassed or otherwise vacuum treated so that the oxygen content will be below 150 p.p.m., then the boron addition should be made after degassing and should be suflicient to provide a boron to nitrogen ratio of about 1.0 to 1.4.

After the composition of the steel melt is adjusted as described above, the steel is then poured into the upper end of an open-ended tubular water cooled continuous casting mold. solidification of the steel is initiated in the mold. A casting having a solidified skin surrounding a molten metal core is withdrawn downwardly from the mold, as entire solidification is effected by means of water sprays located below the mold, as is conventional in the art.

When the slab has been suitably conditioned for hot rolling, if such conditioning is necessary, it is heated to a hot rolling temperature within the range 2100 F. to 23-00 F. according to conventional prior art practices. Thereafter the slab is hot rolled according to conventional practices, with the finishing temperature being of course within the austenitic range, i.e., above about 1500 F., and preferably within the range 1550 to 1650 F. To eifect the aging resistance properties of this invention it is essential that the hot rolled steel be coiled at a temperature above 1100 F. and preferably Within the range 1150 to 1250 F. Although the reason for this limit is not clearly understood, the final product does show large reductions in aging resistance at coiling temperatures below about 1100 F.

After hot rolling and coiling as described above, the steel may be pickled and cold rolled in accordance with conventional prior art practices. For example, the steel may be pickled in either HCl or H acid solution, and then cold reduced by 50 to 75%. Conventional tight-coil, box-annealing for 20 hours at 1250 to 1275 F., as is typical for drawing-quality cold-rolled sheet, is most satisfactory for this steel. We have learned, however, that annealing the cold-rolled sheet at temperatures above about 1300 F. for prolonged periods of time will cause the steel to pick up some nitrogen from the annealing atmosphere, usually HNX gas. This will of course have an adverse effect on the steels aging resistance if sufficient nitrogen is absorbed to offset the preferred boronnitrogen balance. Therefore, the cold rolled sheet should preferably be annealed at temperatures below about 1300" F. if nitrogen containing atmospheres such as HNX are used. After annealing, the cold-rolled sheet is temper rolled in accordance with conventional prior art practices.

The resulting cold-rolled sheet steel product obtained by practicing the above invention will have mechanical properties equal to or superior to prior art aging-resistant sheet steels produced from ingot cast steels. Of most significance is the fact that the improved results are substantially more reproduceable than experiences with prior art processes. To be more specific, conventional aging, as indicated by return of yield point elongation, can be reproduceably retarded for at least days. Strain-aging index values can readily be reproduced within the range 0 to 10%. Furthermore, the aging resistance properties can be maximized to virtually an nonaging characteristic, i.e., strain-aging index values of from 0 to about 2.0% if the preferred hot rolling finishing temperatures of from 1550 to 1650 F. and coiling temperatures of 1150 to 1250 F. are provided.

Although the primary object of our efforts was to develop an aging-resistant steel capable of being continuously cast, we have further learned that this invention is equally applicable to ingot cast steels. Hence, this invention ofliers the further advantage of providing an aging resistant steel which can be produced by either ingot casting or continuous casting operations.

EXAMPLES To exemplify typical experiments within the scope of this invention, three 35-ton basic oxygen process (BOP) heats were prepared and adjusted to the composition shown in Table I. The boron was added as ferroboron, aiming for 0.004% (B/N=2.0) in the first heat, 0.008% (B/N=2.5) in the second heat, and 0.006% (B/N=1.5) in the third.

These three heats were then continuously cast to 7 /2 inch thick by 27 /2 inch wide slabs. The slabs were then processed to cold-rolled and annealed sheets on commercial facilities utilizing conventional practices. A hotrolling finishing temperature of 16301640 was maintained and the coiling temperature was maintained at 1160-1190 F. The head, center and tail of the sheets were then tested for strain-aging and plastic strain ratios. For the strain-aging test specimens, the sheets were strained 5% in tension, aged for four hours at 212 F., then restrained. The percent increase in yield or flow stress as a result of strain-aging (strain-aging index) was then calculated. To test for plastic strain ratio, R specimens from the sheets were carefully measured before and after straining in tension to maximum load. The R (or plastic strain ratio in the longitudinal direction) was then calculated as the ratio of the true width strain to the true thickness strain as is well known in the art. Table I below gives the results of the tests as well as the compositions of the heats. In addition, eighteen heats not containing boron were identically prepared, processed and tested, with the typical results thereof shown at the bottom of Table I. The improved strain-aging index provided by this invention is readily apparent.

TABLE I seen in Table I, the process of this invention had no substantial adverse effect on the steels formability," as measured by the plastic strain ratio, R value. i

Further evidence of the non-strain-aging behavior of properly processed boron-containing special continuous cast steel (exhibiting aging-index values of 0 to 8%, Table I) is given in Table II below which shows no change in tensile properties for two'of the first three steels when tested shortly after temper rolling and after approximately 2 /2 years of storage at room temperature. It is apparent from these results that there had been no deterioration in any of the properties and that there had been no return of the yieldpoint behavior which would be associated with strain-aging.

' TABLE II Tensile Elongation Yield point Yield strength, strength, in 2 inches, elongation K s.i. K s.i. percent percent Results of tests conducted initially, shortly after temper-rolling Results of tests conducted 2% years after above initial ests 23.3 43. 6 42.5 0 22. 8 43. 3 42. 0 0 27. 4 47. 2 40. 2 0 25. 8 46.4 v 40.0 0

1 At 0.2% offset.

In another test, 8 additional non-vacuum-treated 35-ton BOP heats were prepared with varying deoxidation practices. Boron was added to these heats in the form of ferroboron or calcium boride, in amounts falling within Composition, Strain-Aging Characteristics, and R Values for Three Heats Processed in Accordance with this Invention Contrasted to Typical Results for 18 Prior Art Heats Composition, percent Strain- Plasticaging strain index; ratio, Location 0 Mn Si A1. 1 Al 2 B N percent Ry,

Heat Boron-treated heats 0. 002 0. 8 1. 09 1A 0. 002 1. 2 1. 09 0. 002 1. 2 1. 14 0.002 0. 0 1. 14 1B 0. 003 1. 8 1. 11 0.002 8. 0 1. 22 0. 002 2A Center- 0. 046 0. 38 0. 044 0. 007 0. 016 0. 0075 0. 003 0. 5 1, 04 Tail 0. 0083 0. 003 Head 0. 0080 0. 001 2. 8 0. 98 2B Center- 0. 049 0. 38 0. 045 0. 006 0, 010 0. 0070 0. 003 0. 5 1. 03 1 Tail 0.001 4.0 1.00

N on-boron treated heats (typical values) 18 heats Various 0. 030/0. 046 0. 36/0. 47 0. 026/0. 052 003/0. 009 N.A. 17 1. 1

1 Soluble Al. 2 Total Al.

3 Results of tests on specimens oriented in the rolling direction. 4 Not determined.

From Table I above, it can readily be seen that the boron containing heats had substantially improved agingresistance, some of them being so significantly improved and outside the scope of this invention. All 8 heats were continuously cast, processed, and tested asdescribed for the first three heats. The results of the testsare shown as to be considered virtually non-aging. As may also be 75 in Table III following:

TABLE III Boron-Containing Continuously Cast Steels Produced on Commercial-Size Equipment Composition, percent 2 Aging BIN, index, Deoxidation practice 1 B N ratio percent Initial casts 4 6.42 SiMn, 1.36 FeMn, 1. 0. 0015/0. 0014 0. 003 0. 5 16.2 8.32 SiMn, 2.02 FeMn, 1 0. 0042 O. 006 0.7 15.0 8.31 SiMn, 1.56 FeMn, 0.0027 0. 004 0. 7 14.1 8.46 SiMn, 1.67 A11, B) 0.0053 0.005 1.0 9.8 7.06 SiMn, 0.78 Fe 1. 1.62 (13% B) 0. 0050/0. 0065 0. 002/0. 003 1 6/2. 7 0. 4/8. 8 8.04 SiMn, 0.54 FeMn, 1.64 Al, 0.54 FeB (18% B), 0.24 CaB.--- 0.0031 0.004 0. 8 20.4 1.2 FeMn, 6.6 SiMn, 1,8 A1, 1.3 FeB 0.0051 0. 002 2. 5 10. 2 11 Mn, 1.2 FeSi, 1.8 A1, 1.5 FeB 0. 0056 0.002 2. 8 2.2

1 Additions are given in pounds per ton. 2 Sheet cheek analyses. I Amount of boron addition not recorded.

TABLE IV Efleets of Room-Temperature Aging on the Mechanical Properties of a Boron-Treated Retarded-Aging Steel Lapsed Time from Yield Uniform temper Yield Tensile point elonga- Hardrolling, strength, strength, elongation, tion, ness days K s. K s.i. percent percent R30-'1 Table V below exemplifies the effect of varying the boron to nitrogen ratio in degassed steels. Table V shows seven coils prepared from two 300 ton commercial sized heats which were degassed to less than 150 ppm. oxygen and the coils rolled therefrom having differing boron to nitrogen ratios. As the table indicates, boron to nitrogen ratios below 1.4 down to about 1.0, were not detrimental to aging resistance when the steel was degassed to less than 150 p.p.m. oxygen. Although optimum aging resistance was achieved at ratios of from about 1.0 up to 1.70, optimum ductility (not shown in the table) was achieved at boron to nitrogen ratios within the range about 1.0 to about 1.4.

TABLE V Composition and Strain-Aging Characteristics of Cold-Rolled Sheets lirgduced from Boron-Containing Steels Degessed to 150 p.p.m. o xygen The critical eifect of coiling above 1100 F. is shown in Table VI below. For this study, three different pairs of coils were examined. Both coils in each pair were identical in composition being within the preferred limits taught herein and were identically processed except that the finishing and coiling temperatures were varied as shown in the table. For reasons we cannot explain, those coils coiled at temperatures below 1100 F. were far inferior to those coiled at temperatures above 1100 F.

TABLE VI Effect of Hot Rolling Conditions on Strain-Aging Characteristics Strain- Finishing Coiling aging index, temp., F. temp., F. percent Heat:

l Exiting last pass. 2 Entering last pass, actual finishing temperature estimated to be 50 F. lower than temperature listed.

Further extensive tests with 200 ton commercial sized heats, both conventional and vacuum treated, have confirmed the criticality of the boron to nitrogen ratio and coiling temperatures as taught herein.

We claim:

1. A method of producing thin flat rolled steel products having substantial aging-resistance comprising forming a steel melt consisting of 0.0l-0.08% carbon, 0.20- 0.60% manganese, 0.0*30.08% silicon, -0.004-0.015% aluminum with a balance of iron and other usual steelmaking impurities including oxygen and nitrogen; adding up to about 0.01% boron to the steel to provide a boron to nitrogen ratio of 1.4 to 2.5 when the oxygen content is more than about ppm. and a ratio of 1.0 to 1.7 when the oxygen content is less than 150 p.p.m.; casting and forming the steel into a slab form suitable for hot rolling; reheating and hot rolling the slab to hot rolled sheet thickness, thereafter coiling the hot rolled sheet at a temperature above 1100" F.; and fially pickling, cold-rolling and annealing the hot rolled steel in accordance with conventional mill practices.

2. The method according to claim 1 in which said steel is continuously cast to form said slab.

3. The method according to claim 1 in which the steel is hot rolled to provide a hot rolling finishing temperature within the range 1550 to 1650 F., and said coiling temperature is within the range 1150 to 1250 F.

4. The method according to claim 1 in which the steel,

after cold rolling, is annealed at a temperature below 1300 F.

5. The method according to claim 1 in which said boron to nitrogen ratio is 1.4 to 2.0 when the oxygen content is more than about 150 p.p.m., and 1.0 to 1.4 when the oxygen content is less than about 150 p.p.m.

6. A flat rolled steel product characterized by good surface quality, good drawability and exceptional aging resistance consisting essentially of 0.010.08% carbon, 0.200.60% manganese, 0.03-0.08% silicon up to 0.015% aluminum, other usual impurities including nitrogen and oxygen, less than about 0.01% boron but sufficient to provide a boron to nitrogen ratio of from 1.4 to 2.5 when the oxygen content is more than about 150 p.p.m. and a ratio of from 1.0 to 1.7 when the oxygen content is less than about 150 p.p.m., said flat rolled steel product having been hot rolled from a slab and thereafter coiled at a temperature above 1100 F and finally pickled, cold References Cited UNITED STATES PATENTS 3,512,957 5/1970 Brotzmann et al 75-49 2,999,749 9/1961 Saunders et al. 75-58 3,558,370 1/1971 Voni 75--123 I X CHARLES N. LOVELL, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R. 75-123 B 

